PONTINE NUCLEI AND MIDDLE CEREBELLAR PEDUNCLE
As presented earlier, the pons is divided into two portions: the dorsal portion called the pontine tegmentum, and a ventral part referred to as the basilar pons.
The ventral portion of the pons is a massive structure that consists of orderly arranged transverse and longitudinal fiber bundles between which are large collections of pontine neurons called the PONTINE GREY (or GRAY). The longitudinal bundles are (1) corticobulbar, (2) corticospinal and, most important for this point, (3) CORTICOPONTINE FIBERS.
I touched on some of the connections and functions of the cerebellum when discussing the accessory cuneate nucleus (POINT #5) and the inferior olivary complex (POINT # 6). There will also be several lectures on the cerebellum. Right now, you need to know that CORTICOPONTINE fibers convey information from motor related areas of cortex (i.e., the cells of origin) to neurons in the IPSILATERAL pontine grey (pontine grey neurons). More specifically, corticopontine axons convey to the pontine grey neurons information that is used in the planning and initiation of movements. These planning and initiation data are then sent to the CONTRALATERAL cerebellum by another projection called the pontocerebellar tract.
The corticopontine fibers descend from cerebral cortex uncrossed to end upon the pontine grey nuclei. Cells within the pontine grey then project via the MIDDLE CEREBELLAR PEDUNCLE (or brachium pontis) to the CONTRALATERAL cerebellum via pontocerebellar fibers. This cortico-ponto-cerebellar pathway is quantitatively the most important route by which the cerebral cortex can influence the cerebellar cortex. The corticopontine projection carries information that the contralateral cerebellum (via a synapse in the pontine grey) uses to participate in the preparation to move and in the initiation and execution of movements. Much more later!
You know that each CEREBELLAR HEMISPHERE influences the SAME SIDE of the body. Lesions of the corticopontine projection and the basilar pontine grey result in CONTRALATERAL deficits in muscle coordination. This incoordination/ataxia involves both the arm and the leg. In contrast, a lesion of the middle cerebellar PEDUNCLE results in IPSILATERAL deficits in motor coordination of the arm and leg. Why??
What about a Romberg sign following a lesion of the middle cerebellar peduncle? Well, since the cerebellum is not receiving its normal input from the pons, you cant stand with your heels together to begin with, so you could not have a Romberg sign. Same for inferior cerebellar peduncle?
You will hear more about this pathway in the lectures on the cerebellum. Right now, remember, for the cerebral cortex to inform the cerebellum about a movement, two neurons are needed. The first lies in the cortex; its axon (corticopontine) enters the internal capsule (along with corticospinal and corticobulbar axons), passes into the cerebral peduncle at midbrain levels, and enters the basilar pons where it terminates upon pontine grey nuclei. Pontine grey neurons possess axons (pontocerebellar) that cross in the basilar pons and project to the contralateral cerebellar cortex via the middle cerebellar peduncle. The pontocerebellar projection is the primary constituent of the middle cerebellar peduncle, which is also called the brachium pontis. Oh, by the way, do pontocerebellars end as mossy or climbing fibers? (remember there is one source of climbing fibers).